PROJECT SUMMARY/ABSTRACT Prostate cancer (PCa) is now the most frequently diagnosed cancer and the second most common cause of cancer deaths in men in United States. Notably, African Americans (AAs) exhibited 1.6-fold higher incidence and 2.4-fold higher mortality rates compared to European Americans (EAs). Despite multiple socioeconomic factors postulated to explain the observed PCa disparities, higher recurrence and mortality rates remained even after adjustment of socioeconomic status in AAs, suggesting that intrinsic biological differences account for, at least, part of PCa disparities. Our previous genomic studies have revealed that intrinsic genomic differences do exist between AA and EA PCa. These genetic elements, including population-specific and -enriched microRNAs, mRNAs and alternative splicing variants, have been identified and were hypothesized to contribute to the differential PCa properties between AA and EA. In the pilot study, our preliminary results revealed ~2,500 differential splicing (DS) events occurring in AA and EA PCa. Among these DS genes, >70% of the genes were functionally linked to cancer diseases. These results lead to our hypothesis that aberrant mRNA splicing may play a critical but largely unknown role for driving the PCa disparities. Towards this hypothesis, we have performed RT-PCR validations and functional analyses of AA-enriched splice variants (such as PIK3CD, FGFR3, TSC2 and RASGRP2 splice variants) in PCa samples. Our preliminary results confirmed that these AA-enriched variants were highly expressed in AA PCa and contributed to more aggressive cancer phenotypes. In this proposal, we will focus on investigating the expression profiles of PIK3CD long and short splice variants (PIK3CD-L and PIK3CD-S) in a panel of PCa and other solid/hematologic tumor cell lines, and elucidating functional impacts of these splice variants in PCa aggressiveness and drug resistance. Guided by strong preliminary data, we propose to pursue three Specific Aims to character molecular functions of the PIK3CD splice variants underlying PCa aggressiveness: 1) Validate the expression profiles of PIK3CD splice variants in PCa specimens and cell lines derived from AA and EA patients; 2) Determine the functional roles of PIK3CD-S expression in disease aggressiveness and drug resistance in PCa and evaluate the feasibility of PIK3CD- S/PIK3CD-L ratios as potential biomarker; and 3) Screen for the effective therapeutic molecules (small molecule inhibitors, siRNAs and/or splice switching oligos) to reduce the cancer phenotypes in PIK3CD-S overexpressing cells. Collectively, our proposed research will broadly contribute to the field of cancer health disparities by characterizing the molecular signatures of aberrant splice variants in cancers (including PCa and other solid/hematologic tumors), and deciphering the molecular mechanisms of aberrant splicing underlying PCa aggressiveness (i.e. enhanced cell proliferation and invasion) and drug resistance. This proposal will allow us to further design for splice variant- specific siRNAs and screen for SMIs that can effectively inhibit the aggressive form of splice variants in PCa. These findings and efforts may facilitate the development of potential biomarkers and therapeutic strategy to treat aggressive PCa. Finally, the success of this SC1 proposal will set a strong foundation for the PI and his research team to further pursue a NIH competitive grant (i.e. R01 or R21) for future clinically-related studies.